We have demonstrated that environmental cold (0 to -5 C) raises pulmonary arterial pressure in cattle through an increase in both pulmonary blood flow and vascular resistance. Only part of the increase in resistance is due to hypoxia which results from hypoventilation and increased metabolism. Further studies have been proposed to determine the "total basis" for the cold-induced rise in pulmonary vascular resistance and blood pressure. Experiments will be conducted in calves in a large hypo-hyperbaric chamber designed for cooling below OC. Serial measurements of ventilation, end-tidal gases, oxygen consumption, arterial blood gases, cardiac output, blood pressures and skin and body temperatures will be made with the calf confined to a closed-type plethysmograph. The hypoxic vasoconstriction will be removed by pressurizing the large chamber to sea level or below or by regulating oxygen inflow to the plethysmograph so that "other contributing factors" might be more easily examined. These may include increased activity of the sympatho-adrenal system which will be evaluated by catecholamine assays and pharmacologic blockade. The results could be important to both human and animal health.